Certain peripheral units must of course receive from a computer words of data having a predetermined size. However, very often the words supplied by the computer have a size which is incompatible with the size of the words which the peripheral unit can deal with.
This is the case, for example, with certain graphic printers, which must receive simultaneously data corresponding to a certain number of consecutive lines: in the computer each line is represented by a sequence of words in which each word corresponds to two juxtaposed dots of a line of the graphic image traced by the printer. Words of data whose size is different from that of the words of the computer must therefore be delivered to the peripheral unit (a printer in the example under consideration), the data of each word received by the peripheral unit corresponding to superposed dots of a column of the graphic image.
In contrast, in certain other applications the peripheral unit must transmit to the computer words whose size does not correspond to that of the words which the computer can accept.
The known methods, which enable, for example, a computer to deliver to a peripheral unit words such that the computer handles them and which enable the peripheral unit to receive them as it requires them, are of two kinds:
in one of the methods, data coded in a manner directly compatible with the peripheral unit are recorded in the computer memory, and processing is carried out directly in that format. If the sizes of the words are incompatible, this supposes that either a number of words of the computer are used for one word of the peripheral unit, or on the contrary, a portion of the computer word is used for a word of the peripheral unit. This forms a first disadvantage. Moreover, the processing may be inapplicable to such data coding. Thus, for example, in the case of a printer the computer produces a sequence of words for each line of the graphic image, and the peripheral unit must receive words corresponding to superposed points of a column of such graphic image. One of the known methods consists in representing the image to be obtained internally, in the computer, by words representing dots of the same column. This type of method is incompatible with sceen-type memories, since the screen is swept horizontally. Neither is it compatible with image-compressing methods, which favor the horizontal direction;
another kind of known method uses the computer itself to transcode the words. However, the usual instructions of computers are ill-adapted to such processing, so that processing lasts too long. For example a modern microprocessor takes 4 seconds to return an image of 4 million dots. Such a duration is incompatible with mass printing as allowed, for example, by a laser printer. Moreover, the data output must take the form of words of the size demanded by the printer. For example, for a telecopying machine operating by the standard of group 4, the computer must handle words of 8 bits; this is inefficient and demands the full-time mobilization of the computer while the printing of a whole page lasts.